Non-contact temperature instruments allow measuring the temperature of an object at a distance and are quick to respond. These operating features are particularly helpful when measuring the temperature of an object in a harsh or dangerous environment where physical contact is not an option. Such instruments generally operate by sensing the energy emitted from objects at a temperature above absolute zero in which the radiant infrared energy emitted by the object is proportional to the fourth power of its temperature.
To develop a measurement, some devices use a shield, often called a chopper to expose a sensor or detector, alternately exposing and blocking the target object, creating a modulated signal.
Many optical sensors or detectors, such as lead sulfide detectors, exhibit long term drift in their responsiveness, that is, the output as a function of incident radiation may change over time. Devices using such detectors must be periodically recalibrated using a source of known brightness, such as an incandescent bulb. One such method of calibration requires that the device is removed from service, exposed to the incandescent bulb, and then manually recalibrated. However, this requires that the measurement of the target object be interrupted during the calibration period. If the device is being used in a process control environment, either the process must be halted or the process must run out of control during the calibration period. Waiting for the incandescent bulb to warm up and stabilize may introduce further delays in the calibration process.
One attempt at automated self-calibration used an incandescent bulb arranged so that the chopper would expose the sensor to the target object, block the target object, expose the sensor to the bulb, block the target, and again expose the sensor to the target object. This approach requires the incandescent bulb to be run continuously, introducing its own brightness drift over time as a source of error. Moreover, when operated in this manner, the device is only measuring the target object one quarter of the time, reducing both the signal-to-noise ratio and its responsiveness to changes in target radiation.